It is known that aquarium water is rich with nutrients provided by the breakdown product of the fish's metabolism and bacterial breakdown of waste products. The waste aquarium water is found to be highly beneficial as a fertilizer for house-plants. Such nutrient-enriched water can be immediately used for watering plants without aging or special preparation.
To utilize such valuable quality of the aquarium water, it would be preferable to have a cap enclosing the open mouth of a readily-portable jar-aquarium so a small aquatic animal would not slip out while watering house plants directly from the aquarium. Such watering of plants would become easier if the cap was designed for controlled pouring water into a flower-pot.
It would also be highly desirable to have a dual-purpose cap adapted to simplify changing of aquarium water by allowing pouring water out and into the aquarium without taking the cap off.